Since 1945, after World War II, the United States has made a concerted effort to prevent problems like Hitler in Europe. Any totalitarian regime, especially communist or socialist, is an enemy or at least closely watched. The same is true for any extremists or unstable governments, such as the Middle East.

I would say the two words mentioned above by brettd, interventionist and containment, are a pretty good summary of foreign policy since 1945. We have made ourselves the keepers of the peace around the world.

That's a very large question, and impossible to answer in so short a space. In general, US foreign policy during that time, in the post-World War II era, American foreign policy has been interventionist. This means that our foreign policy during that time was built around giving foreign military and economic aid to allies, as well as stationing troops and becoming involved in foreign wars. Mostly, this was because we were heavily committed in the Cold War against the Soviet Union, and we wished to stop the spread of communism to other countries. Other than interventionist, you can also describe our foreign policy between 1945 - 1991 as "containment".

This question is really too broad for a good answer in the kind of space we have here.

I would suggest that you are going to need to break this up, at the very least, into looking at the Cold War era and then at the era after the Cold War. You can argue that the post-2001 era is actually separate from the time between 1990 and 2000 as well.

During the Cold War, US foreign policy of course revolved around the containment of communism. As far as analysis, you could say that the policy was effective in that communism more or less died out. At the same time, you can criticize US foreign policy for its lack of attention to human rights -- it was willing to support pretty much anyone as long as they weren't communist.

After 2001, US foreign policy has been mostly concerned with fighting terrorism. To this point, foreign policy has been caught up in the wars in Iraq and Afghanistan. These wars seem not to be working, but I think that it is too early to definitively analyze whether current foreign policy is working.